Cellulosic material



This invention relates to improving the properties of cellulosic material and in particular to preservation of the white or colorless condition of regenerated cellulose.

Regenerated cellulose has many fields of possible use where it is unavoidably subjected to heat during preliminary processing operations. The heating conditions referred to are those in which the temperature of the material itself is elevated substantially above 110- 120 F., i.e. 200 F. and higher, as distinguished from usual drying operations in which the wet bulb effect in the material keeps its temperature depressed even though the temperature of the air may be quite high. When regenerated cellulose itself is heated under these conditions it tends to discolor, that is, turn yellowish. This discoloration is a serious disadvantage due to the decrease in aesthetic appeal and consumer acceptability. Accordingly, use of regenerated cellulose, whether in the form of film or fiber, has been seriously limited in many applications where it would be acceptable but for its susceptibility to discoloration.

For instance, it has been proposed to replace cotton with rayon, i.e. regenerated cellulose, in many uses because of the greater uniformity of rayon, its lower cost, and the fact that rayon need not be subjectedto wet processing or cleaning operations. However, here again discoloration, in this case during application of the heat associated with steam sterilization, has rendered the rayon substantially unacceptable.

One object of the invention is to modify the property of regenerated cellulose so that it will resist changes in whiteness and colorlessness upon heating.

A preferred object is to devise a treatment for regenerated cellulose fibers such that on subsequent heating, e.g. steam sterilization, their tendency to discolor will be substantially decreased.

In this invention, regenerated cellulose fibers, prior to being subjected to heat (for example, steam sterilization or oven heating for other purposes), are treated with any of certain soluble metal salts which are colorless and which form aqueous solutions having approximately neutral pH described below. The metal cation is thought to be the active principle. The metals contemplated within the inventive concept include those of group IA of the periodic table having atomic number of at least 11, and those of group IIA of the periodic table having atomic number of at least 20. The metallic ion remains combined with the regenerated cellulose and will be present in the regenerated cellulose fibers during the subsequent heat treatment. In this way any small change in color occurring during the heat treatment is markedly less than would occur but for the presence of the metallic ion. Certain significant characteristics of the invention method and product are developed more fully below.

Rayon, which has been. regenerated, coagulated, washed, bleached and dried, has an extremely high degree of whiteness. However, a major portion of this whiteness is lost when the material is subsequently subjected to elevated temperatures such as 200 F. and above in dry or wet atmospheres. In particular, at 240 F. in saturated steam lbs. per square inch gauge pressure) a noticeable and measurable development of yellow to yellow-brown color is observed in the rayon in min ice - by D. B. Judd, Specification of Color Tolerances at the National Bureau of Standards, Am. J. Psychology 52, 418 (1939); R. S. Hunter, Photoelectric Tristimulus Colorimetry with Three Filters, J. Opt. Soc. Am. 32, 509

(1942) [National Bureau of Standards Circular C429 (1942)]; R. S. Hunter, A Multipurpose Photoelectric Reflectometer, J. Opt. Soc. Am. 30, 536 (1940) [J.

Research NBS 25, 581 (1940) NBSRP 1345]. It IIlCflS'.

ures a factor (+b) which is related to the psychological impression of yellowness. been atfected by the invention. For example, a highly purified bleached rayon may have a +bvalue of about 3.5 when dried under usual conditions. If subsequently treated at 240 F. in saturated steam for 20 minutes,

a (+b) value of 5.9 to 6.3 is observed. The change from 3.5 to 5 .9 or more is immediately apparent to the normal eye in white light. By treating the regenerated cellulose fibers with one of the metallic ions according to the invention, the increase in +b value becomes substantially smaller, thereby indicating a substantial decrease in yellowing. The degree of stabilization obtained is particularly noticeable if the regenerated cellulose fibers are treated with cations in the wet gel state, that is, following regeneration and purification but before being initially dried (for example, after the bleach). However, substantial color stability may also be imparted by treating the regenerated cellulose fibers after initial drying The same eifect is noted for regenerated cellulose film as for rayon, discussed above, except that in the case of film it is usually sought to retain original clarity and colorlessness.

According to the invention, the metals utilized in the form of their cations for treatment of regenerated cellulose are the group IA metals of the periodic table having atomic number of at least 11, for example sodium or potassium; or group IIA metals having atomic number greater than 20, such as calcium, strontium or barium. The metals are generally applied in the form of an aqueous solution to the regenerated cellulose fibers. The solutions are utilized in amount and have cationic concentration sufficient to impart desired color stability characterthe weight of dry cellulose.

istics to the regenerated cellulose fibers; that is, the amount of dissolved salt or the salt concentration is such as to form a solution having a pH as set forth hereinafter. The contact time between the regenerated cellulose and the solution may suitably be about five minutes, or more or less depending upon the particular conditions. Excess solution is squeezed from the fibers, for example between pressure rolls, leaving the desired amount of solution on the fibers, for example about 100% solution based on Although the solution concentration is subject to wide variation, acceptable concentrations may be found within the approximate range 0.03 gm. ions per liter to 0.05 gm. ions per liter. 7

In order that the desired objects of the invention may be. realized the cation'and the anion associated therefibers with the above described metallic ions a major proportion of the carboxyl groups which areinvariably found on the cellulose chain are converted to salt groups of the particular cation utilized. This is thought to impart the quality of color stability to the molecule.

It is this property which has,

. usedin Example II.

The following examples are intended to illustrate rather than limit the invention.

Example I Samples of regenerated cellulose (cellophane) each 4" to 6" wide and 4 feet long were washed in distilled water and then passed through a bath of a particular aqueous salt solution three times back and forth. The contact time of the cellophane with the bath was about to 10 seconds. The sample was then hung up to dry at room temperature. The pH and concentration of each solution was determined. After drying, the samples were heated for 3 days in a drying oven at 100 C. The color of the cellophane at the end of this time was observed and compared with the original untreated material. The results are presented in the following table.

Dull regenerated cellulose fibers (rayon containing TiO pigment) of 1% staple length and 1.5 deniers per filament, crimped and bleached, was selected for test. The rayon had been finished with a lubricating finish containing non-ionic detergent. The rayon had been dried prior to the test. Separate samples of the rayon, 6 grams each, were soaked 2 hours in 500 cc. of solutions of sodium sulfate and barium chloride respectively, 1 normal solution in each case. A third portion was treated with distilled water as a control. Duplicate samples were treated with each of the solutions. The rayon was immersed in the solution and squeezed out by hand until it contained approximately 300% by weight of solution based on dry fiber weight. The samples were dried in circulated hot air at 200 F. After drying, samples were fluffed on U.S.D'.A. fiber blender (or card) (to remove knots that might cast shadows and introduce error in color measurement). One sample was thereafter subjected to steam sterilization for 20 minutes in saturated steam at 240 to 250 F. The other sample was aged in an oven at 100 C. for 63 hours. Percent reflectance and +b values were measured on the Hunter meter in each case. The percent reflectance is a determination of the reflection of light from the sample as compared with reflection from a standard white surface. Results are presented in the following table.

The data show a notable decrease in yellowing for the samples treated with salt as compared with the untreated control (5.4-5.2) for both steam sterilization (5.0-5.4) and oven aging (4.6-4.9). The reflectance'values remained substantially constant, since'the minor changes measured by the instrument in this and subsequent examples are not perceptible to the eye.

Example 111 The procedure described in' Example 11 was repeated for similar type rayon using solutions of calcium nitrate and barium chloride in concentrations lower than those The control sampleswere not washed prior to heating. 'The oven aging was carried Steam Sterilization 0 vcn Aging Concentration (Normality) Salli Reflectance, percent It is noted that the untreated control had a relatively high yellowness factor of 6.6 (steam sterilization) and 5.9 (oven aging), whereas the salt-treated materials were substantially lower, in the range of 5.9 to 6.4 in the case of steam sterilization and 4.9 to 5.4 in the case of oven aging.

Example IV 200 grams of dull rayon of the type described in Ex ample II, but which had not previously been initially 'dried, was selected for experiment. The wet gel, un dried startingmaterial had previously been regenerated, washed, bleached and treated with finish. .It was squeezed out by hand as dry as possible. 200 grams of the squeezed material was soaked for 4 hours in one liter of 0.02 normal aqueous barium chloride solution. The solution was drained off and the sample dried in circulating hot air. Its weight was 42 grams. The control sample, consisting of another 200 gram portion of the squeezed material, was merely dried in circulating air without salt treatment. The control and salt-treated samples then were subjected to two successive steam sterilizations, as described in previous examples. Reflectance and +b measurements appear in the following table.

Reflectance, +b

percent It is noted that the difference between the control (7.1) and the salt-treated material (6.2) is impressive.

Example V Goncen- Reflectance, Salt tration percent +b (Normality) Contro 86.6 6.4 (321012 0.10 N 88. 2 5. l B11012 0.02 N 85. 5 5. 0

The control sample, it will be noted, had a relatively high yellowness factor of 6.4, whereas the salt-treated materials had substantially lower yellowness factors of 5.0 to 5.1. Example VI The salt-treated and sterilized samples from Example V were subjected to an aging test in an oven in dry air at F. Measurements of reflectance and yellowness were made in the usual manner after 0, 3, 7 and 10 days time of treatment. Results are given below.

It is seen that in every case the yellowness values were substantially higher for the control than for the samples treated with salt according to the invention.

Example VII Dried samples of bright and dull rayon of the type tested in the foregoing examples were soaked for 4 /2 hours in aqueous sodium chloride solutions of various normalities. The samples then were air-dried. A portion of each sample was steam-sterilized (240 F., 20 minutes, saturated steam) and the color determined. The samples were then re-sterilized under the same conditions as before and the color re-determined. Another portion of each sample (not sterilized) was heated in an oven for 10 days at 140 F. and the color was determined. The control sample was not treated with salt nor was it washed with water. It was merely subjected to the steam sterilization or oven heating described for the salttreated samples. In the following table the results obtained from color measurements on the control and on the samples treated with the various salt solutions are presented.

sterilized Sterilized Oven-Heated Goncen- Once Twice Sample tration Rei'l. +1) Refl. +b Refl. +b

Bright rayon- 0.01 N--- 89. 3 4. 8 85.4 5.9 91.4 3. 6 Do .10 N- 87. 8 4. 9 S5. 7 6.0 90. 3. 4 86. 3 4. 7 83.1 5. 7 90.8 3. 5 86. 7 5. 7 85. 7 6. 7 92. 2 4. 3 88.1 4. 9 85.1 6. 3 91. 5 4.0 87. 7 5. 2 84. 6 6. 6 90. 9 3. 7 86.5 5. 0 82. 7 6.6 90. 3 3. 6 89.1 5. 7 85. 4 6. 7 91. 7 4. 4

Example VIII Samples of bright rayon of the type described above were soaked for minutes in aqueous calcium chloride solutions of 0.002 and 0.02 normality, dried in hot air and sterilized with steam by the procedure described above. Reflectance and +1; measurements were made and are presented in the following table.

Concentration Reflectance +6 Example IX sterilized by the standard method and color reflectance measurements were made. The results appear in the following table.

Treatment Reflectance +b 85. 9 7. 3 O1 88. 9 6. 8 89. 2 6. l BaGlg 87. 9 6. 9 NaCl 87.9 6.9

The high degree of discoloration of the control sample was indicated by the high +b value (7.3) which is to be contrasted with the substantially lower values (6.1 to 6.9) for the samples treated with salt solutions.

Although the embodiments of the invention have been described in detail in the foregoing description, this material has not been presented in a limiting sense. Accordingly, it is desired that the appended claims be construed broadly enough to cover all modifications and equivalents within the spirit of the invention.

The claims are:

1. As an article of manufacture, regenerated cellulose fibers resistant to discoloration when said fibers are subjected to elevated temperatures and containing combined with the regenerated cellulose a metal selectd from the group consisting of metals of group IA of the periodic table and having an atomic number of at least 11 and the metals of group IIA of the periodic table having an atomic number of at least 20, said fibers having been formed by impregnating with an aqueous solution having a pH of between about 6 and 8 of an ionizable, colorless, watersoluble salt of said metal, the anion of the salt being selected from the group consisting of sulfate, bisulfate, chloride, bromide, acetate and nitrate ions, said solution being formed by dissolving in water a sufficient amount of said ionizable salt to form the solution havinga pH of between about 6 and 8, drying said fibers in the presence of said metallic ion, the dried regenerated cellulose fibers having been subjected to steam sterilization subsequent to drying.

2. The method offorming regenerated cellulose fibers resistant to discoloration when said fibers are subjected to elevated temperatures which comprises impregnating said fibers with an aqueous solution having a pH of between about 6 and 8 of an ionizable, colorless, watersoluble salt whose cation is a metal selected from the group consisting of the metals of group IA of the periodic table and having an atomic number at least 11 and the metals of group IIA of the periodic table having an atomic number at least 20 and whose anion is selected from the group consisting of sulfate, bisulfate, chloride, bromide, acetate and nitrate ions, said solution being formed by dissolving in water a sufficient amount of said ionizable salt to form the solution having a pH of between about 6 and 8, drying said fibers in the presence of the metallic ion of the salt, and thereafter subjecting the dried fibers to a steam sterilization treatment.

3. The method of forming regenerated cellulose fibers resistant to discoloration when said fibers are subjected to elevated temperatures which comprises impregnating said fibers while in their wet gel state with an aqueous solution having a pH of between about 6 and 8 of an ionizable, colorless, Water-soluble salt whose cation is a metal selected from the group consisting of the metals of group IA of the periodic table and having an atomic number at least 11 and the metals of group IIA of the periodic table having an atomic number at least 20 and whose anion is selected from the group consisting of sulfate, bisulfate, chloride, bromide, acetate and nitrate ions, said solution being formed by dissolving in water a sufiicient amount of said ionizable salt to form the solution having a pH of between about 6 and 8, drying said 8 Drew July 25, 1939 Lessig Apr. 11, 1944 FOREIGN PATENTS Great Britain Mar, 2, 1936 OTHER REFERENCES 7 Ott, Emil: High Polymers, vol. 5, Cellulose and Cellu lose Derivatives, 1943, pages 113-118. 

1. AS A ARTICLE OF MANUFACTURE, REGENERATED CELLULOSE FIBERS RESISTANT TO DISCOLORATION WHEN SAID FIBERS ARE SUBJECTED TO ELEVATED TEMPERATURES AND CONTAINING COMBINED WITH THE REGENERATED CELLULOSE A METAL SELECTED FROM THE GROUP CONSISTING OF METALS OF GROUP IA OF THE PERIODIC TABLE AND HAVING AN ATOMIC NUMBER OF AT LEAST 11 AND THE METALS OF GROUP IIA OF THE PERIODIC TABLE HAVING AN ATOMIC NUMBER OF AT LEAST 20, SAID FIBERS HAVING BEEN FORMED BY IMPREGNATING WITH AN AQUEOUS SOLUTION HAVING A PH OF BETWEEN ABOUT 6 AND 8 OF AN IONIZABLE, COLORLESS, WATERED FROM THE GROUP CONSISTING OF SULFATE, BISULFATE, CHLORIDE, BROMIDE, ACETATE AND NITRATE IONS, SAID SOLUTION BEING FORMED BY DISSOLVING IN WATER A SUFFICIENT AMOUNT OF SAID IONIZABLE SALT TO FORM THE SOLUTION HAVING A PH OF BETWEEN ABOUT 6 AND 8, DRYING SAID FIBERS IN THE PRESENCE OF SAID METALLIC ION, THE DRIED REGENERATED CELLULOSE FIBERS HAVING BEEN SUBJECTED TO STEAM STERILIZATION SUBSEQUENT TO DRYING. 